This invention generally relates to fuel cells that rely upon electrochemical reactions to produce energy, more particularly to direct oxidation organic fuel cells which employ oxidation-reduction reactions at electrode surfaces to produce energy from an organic fuel provided in the form of an aerosol.
Fuel cells employing organic fuels are extremely attractive for use in both stationary and portable power applications, in part because of the high specific energy of the organic fuels. Moreover, fuel cells use an oxidation/reduction reaction instead of a combustion reaction, typically producing clean exhaust gases comprising mostly carbon dioxide and water as well as electrical energy.
Previously, some fuel cells used a xe2x80x9creformerxe2x80x9d to convert the organic fuel to hydrogen gas in the fuel cells. Direct oxidation liquid feed fuel cells do not, however, require such fuel processing steps, and therefore offer a considerable weight and size advantage over indirect gas feed xe2x80x9creformerxe2x80x9d fuel cells. In addition, the fuel concentration in a liquid feed is generally much greater than in a gas feed, improving mass transfer at the electrode and increasing cell performance.
Liquid feed fuel cells do have some negative features, including crossover of the fuel to the cathode, resulting in reduced fuel efficiency and decreased fuel cell performance. Crossover may be reduced by using a dilute solution of the organic fuel in an electrolyte such as water; however, this creates increased mass transfer resistance at the anode that may degrade cell performance. In addition, water may also cross over to the cathode in large quantities, which may cause cathode flooding, reducing cell performance and leading to excessive water loss.
The art continuously searches for methods to decrease the size and complexity of fuel cells, and to increase their fuel conversion efficiency, operating voltage, power output and operating performance. In particular, the art searches for methods to increase fuel efficiency and fuel cell operating performance by reducing fuel and water crossover without increasing mass transfer resistances at the fuel cell electrodes.
The present invention provides a direct oxidation fuel cell in which the fuel is provided in the form of an aerosol of liquid fuel droplets suspended in a gas. The fuel cell comprises an anode, a cathode and an electrolyte. The aerosol is formed in an aerosol generator. The aerosol generator may use a variety of methods for producing the aerosol. Suitable methods include atomization of a liquid into a gas, or cooling a superheated mixture of fuel vaporized into a gas below the boiling point of the liquid to nucleate droplets of liquid fuel suspended in the gas. Suitable atomization methods include orifices, single fluid nozzles, two fluid nozzles, rotary atomizers, and ultrasonic atomizers.
In one embodiment, the aerosol is formed in an aerosol generator situated within the anode chamber of the fuel cell. In a preferred variation of this embodiment, the aerosol generator comprises a plurality of atomizers situated at the inner surface of a flow field element, fed with liquid fuel via a conduit, and acting to uniformly distribute liquid fuel droplets over the surface of the fuel cell anode. The atomizers may be selected from any number of fluid atomization devices, including orifices, single fluid nozzles, two fluid nozzles, rotary atomizers and ultrasonic atomizers.
In another embodiment, the aerosol is formed in an aerosol generator external to the anode chamber of the fuel cell and fed to the anode chamber via a duct. In one form of this embodiment, the aerosol is formed by heating the liquid fuel to a temperature above the boiling point of the liquid in the presence of a gas to form a superheated vapor, and subsequently cooling the superheated vapor to a temperature below the boiling point of the liquid fuel, thereby forming a suspension of liquid droplets in the gas. In another form of this embodiment, the aerosol is formed by atomization of the liquid into the suspending gas. The atomizers may be selected from any number of fluid atomization devices, including single fluid nozzles, two fluid nozzles, rotary nozzles and ultrasonic atomizers.
In a third embodiment, the aerosol is formed externally to the anode chamber of the fuel cell, fed to a particle size conditioner situated between the aerosol generator and the anode chamber, and subsequently fed to the anode chamber via a duct. In this embodiment, the particle size conditioner is capable of separating liquid fuel droplets based upon particle size, admitting only droplets having a preferred size distribution to the anode chamber.
In a fourth embodiment, the aerosol fuel exits the anode chamber of the fuel cell through an anode chamber exit vent and enters a fuel droplet recovery unit which acts to separate liquid fuel droplets from the suspending gas. Suitable droplet recovery units include filters, porous membranes, packed beds or electrostatic precipitators. The liquid fuel recovered in this manner may subsequently be recycled to the aerosol generator for reuse in the fuel cell.
In another embodiment, the aerosol feed fuel cell anode comprises an oxidation catalyst supported on a backing layer. In a preferred embodiment for high power density operation of the fuel cell, the backing layer is porous or readily wetted by the liquid fuel. In a variation of this preferred embodiment, the aerosol feed fuel cell further comprises a flow field element having a surface exposed to the anode, wherein the exposed surface of the flow field element is treated to make it hydrophobic, thereby directing coalesced liquid fuel into the zone of reaction on the anode surface.
Yet another embodiment pertains to a method of generating energy using an aerosol feed direct oxidation fuel cell. In a preferred variation of this embodiment, the aerosol feed is formed discontinuously by pulse atomization.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.